The present invention relates to centrifugal blood pumps. In particular, the present invention relates to centrifugal blood pumps that remove accumulated gas.
Delicate surgical procedures require that the site of surgery remain motionless. This requirement made early heart surgery difficult as interrupting the heart's pumping action for the required length of time was invariably fatal.
Traditional heart surgery is carried out with the aid of a “heart/lung machine.” With the heart/lung machine in operation, the patient's heart is stopped while the surgeon performs the delicate surgery required to repair the ailing heart. The two fundamental parts of the heart/lung machine are a blood pump that takes the place of the arrested heart, and an oxygenator that replaces the patient's lungs during the surgical procedure. The heart/lung machine also includes filters, blood reservoirs, and plastic tubing as required to connect the several parts of the bypass circuit.
Although the mortality and morbidity of heart/lung bypass surgery has been greatly reduced over the past several years, hospital stays of two weeks and gradual recoveries of over six months are common. Many of the bad side-effects of heart/lung bypass surgery are thought to result from prolonged blood contact with the various parts of the heart/lung machine.
A new technique for heart surgery has been developed and is generally referred to as “surgery on the beating heart.” In this technique, a stabilizing device is commonly used to hold steady the portion of the heart that is being addressed by the surgeon. A heart/lung machine is not required, because the heart and lungs function normally throughout the procedure. The claimed advantages for this technique include reduced hospital stay, reduced hospital cost, and fewer side-effects such as mental deficit. It is claimed that all of these advantages are the result of reduced blood trauma by elimination of blood contact with the components of the heart/lung machine.
Beating-heart surgery is most commonly used for coronary artery bypass procedures. The procedure is not without problems both for the surgeon and the patient. First, the most commonly used stabilizing device consists in part of a series of small suction cups that grasp the portion of the heart being stabilized. The relatively high vacuum required to grasp the heart may result in blood blisters at the site of the suction cups. Second, since the heart is pumping and the coronary arteries filled with blood, the surgeon must contend with spurting from the coronary artery during the grafting procedure. Third, the cost of disposable devices is comparable to that required for conventional open-heart surgery;
The component of the heart/lung machine that is most suspect for causing blood trauma is the oxygenator. This is typically a device containing hundreds of hollow plastic fibers. During the heart/lung bypass surgery, the patient's blood passes over the outside surface of the fibers, while oxygen is passed through the fibers. Gas exchange imitates the function of natural lungs, but unlike the natural lungs, the oxygenator fibers are made from a plastic material and must have a large surface area in order to oxygenate the blood and to remove carbon dioxide from it.
Traditional heart/lung bypass procedures require placement of a cannula in the vessel (the vena cava) where oxygen-depleted blood is returned to the heart. Because the patient is on an operating table that is at a higher level than the heart/lung machine, a siphoning action draws the venous blood to the heart/lung machine. Air is routinely entrained with the blood, and if not removed, could have fatal consequences for the patient. Therefore, the blood/air mixture is typically directed to a reservoir where the air escapes into the operating room.
New designs for “minimal” heart/lung machines have been recently introduced. These minimal machines are typically “closed” circuits, placed at the level of the operating table to minimize the length of tubing required for the bypass circuit. Because the closed circuit has no inherent means of dealing with entrained air, a special “air removal” feature must be designed into the system. Typically, membrane filters are incorporated into the “minimal” circuit for this purpose. However, the pore size of these membranes is restricted to a diameter that will allow cellular elements of the blood to pass through. This requirement can compromise the membrane's ability to trap entrained air.